The most productive clade of eukaryotic algae, the diatoms, demonstrate a remarkable life cycle in which an inevitable cell size decline must be counteracted by sexual reproduction, through differentiation into a unique cell type called the auxospore. During the last 190 million years of evolution, a large range of sexual cell types and reproductive strategies have emerged within diatoms. Yet, the genetic programs controlling these life cycle stages are poorly understood. Moreover, observations of sex in natural samples are notoriously rare. In this project, I will generate a phylum-wide atlas of gene expression during the different stages of sexual development, covering seven different species. Next, I will use comparative genomics to identify and prioritize sexual genes that are either characteristic of a single phylogenetic clade or conserved in all diatoms. Additionally, I aim to distinguish cell fates based on their transcriptional fingerprint by pioneering single-cell RNA sequencing for diatoms. To functionally characterize cell type-specific genes, fluorescent reporter lines will be generated. Finally, I will apply conserved genetic biomarkers to monitor global marine metatranscriptomes and identify the environmental conditions promoting sex. In pursuit of new insights into the diversity of life cycle regulation in unicellular eukaryotes, this project will contribute new techniques and cell type-defining genes that will play a critical role in future studies.